Disk drives are commonly used in workstations, personal computers, laptops and other computer systems to store large amounts of data in a form that are readily available to a user. In general, a disk drive comprises a magnetic disk that is rotated by a spindle motor. The surface of the disk is divided into a series of data tracks. The data tracks are spaced radially from one another across a band having an inner diameter and an outer diameter. Each of the data tracks extends generally circumferentially around the disk and can store data in the form of magnetic transitions within the radial extent of the track on the disk surface.
A head includes an interactive element, such as a magnetic transducer, that is used to sense the magnetic transitions to read data, or to conduct an electric signal that causes a magnetic transition on the disk surface, to write data. The magnetic transducer includes a read/write gap that positions the active elements of the transducer at a position suitable for interaction with the magnetic transitions on the surface of the disk, as the disk rotates.
As known in the art, the magnetic transducer is mounted by the head to a rotary actuator arm and is selectively positioned by the actuator arm over a preselected data track of the disk to either read data from or write data to the preselected data track of the disk, as the disk rotates below the transducer. The head structure includes a slider having an air bearing surface that causes the transducer to fly above the data tracks of the disk surface due to fluid currents caused by rotation of the disk.
Whenever data are either written to or read from a data track, the transducer gap of the head must be centered over the centerline of the magnetic transitions of the data track where the data are to be written or from where the data are to be read, to assure accurate transduction of the transitions representing data. If the head is off-center, the head may transduce transitions from an adjacent track. A servo system is typically used to control the position of the actuator arm to insure that the head is properly centered over the magnetic transitions during either a read or write operation. In a known servo system, servo position information is recorded on the disk surface between written data blocks, and periodically read by the head for use in controlling the position of the actuator arm. Such a servo arrangement is referred to as an embedded servo system.
In modern disk drive architectures utilizing an embedded servo, each data track is divided into a number of data sectors for storing fixed sized data blocks, one per sector. Associated with the data sectors are a series of servo sectors, generally equally spaced around the circumference of the data track. The servo sectors can be arranged between data sectors or arranged independently of the data sectors such that the servo sectors split data fields of the data sectors, as is well known.
Each servo sector contains magnetic transitions that are arranged relative to a track centerline such that signals derived from the transitions can be used to determine head position. For example, the servo information can comprise two separate bursts of magnetic transitions, one recorded on one side of the track centerline and the other recorded on the opposite side of the track centerline. Whenever a head is over a servo sector, the head reads each of the servo bursts and the signals resulting from the transduction of the bursts are transmitted to, e.g., a microprocessor within the disk drive for processing.
When the head is properly positioned over a track centerline, the head will straddle the two bursts, and the strength of the combined signals transduced from the burst on one side of the track centerline will equal the strength of the combined signals transduced from the burst on the other side of the track centerline. The microprocessor can be used to subtract one burst value from the other each time a servo sector is read by the head. When the result is zero, the microprocessor will know that the two signals are equal, indicating that the head is properly positioned.
If the result is other than zero, then one signal is stronger than the other, indicating that the head is displaced from the track centerline and overlying one of the bursts more than the other. The magnitude and sign of the subtraction result can be used by the microprocessor to determine the direction and distance the head is displaced from the track centerline, and generate a control signal to move the actuator back towards the centerline.
In addition to the bursts of magnetic transitions straddling the centerline, each servo sector contains an Address Mark (AM) field and a Gray code field. The AM field is an asynchronous, absolute timing reference that identifies the beginning of a servo sector and provides the basis for locating the other fields of the servo sector, including the positioning bursts discussed above. The Gray code field contains coded information that indicates the track number where the servo sector is located. This information is used to determine the radial position of an actuator during, e.g. a seek operation to locate a particular data sector, by providing a unique identification for each data track on the respective disk surface.
As should be understood, the reliability of servo signal detection is a significant concern to disk drive designers. In order to insure that the servo sectors are properly processed, the AM field must be precisely and reliably detected to accurately locate the Gray code field and servo bursts for head track location and centerline position control. The Gray code field must also be precisely detected for a definitive indication of radial location.
To that end, the AM and Gray code fields of each servo sector are typically recorded as a series of dibits. The magnetic transitions on a disk surface have a certain magnetic polarity, i.e., a north pole or a south pole. A dibit utilizes adjacent pairs of alternating north pole/south pole transitions. When transduced by a head, a dibit results in two electrical peaks having a predetermined relationship to one another as a function of the spacing of opposite magnetic polarity transitions. The electrical peaks comprise a positive electrical peak followed by a negative electrical peak, or vice versa. Advantage can be taken of the dibit detection because specific peak pair detections, such as a positive peak followed after a certain time by a negative peak, can be more accurately distinguished from random single peaks caused by noise in the system.
Disk drives typically include a qualification circuit that receives signals transduced by a head. The qualification circuit compares the strength of each signal peak to a threshold value and provides an output indicative of detection of a transition when a detected signal peak at least equals the threshold value. In this manner, only peaks most likely indicative of data stored on the disk surface are passed through the disk drive for processing. False peaks caused by noise in the circuit are generally of a magnitude below the threshold value, and are not passed through to the disk drive. The same qualification circuit is ordinarily used to qualify detected transitions representing both data and servo information such as AM and Gray code field information.
A shortcoming of present day disk drives is that circuitry utilized to qualify single signal peak detections does not separately qualify a dibit. In other words, the circuitry is designed to qualify each separate peak without regard to a relationship to another peak as exists in a dibit arrangement. Accordingly, a detected peak that exceeds the threshold value is output by the qualification circuit regardless of whether the single peak is meant to be servo information encoded as a dibit.
Such an arrangement degrades the accuracy of dibit detection because the dual peaks are not separately qualified. Each single strong false peak can be passed to the servo system and incorrectly detected as AM or Gray code information. As should be understood, the inclusion of false peak information in a servo operation renders the AM and Gray code detections inaccurate, resulting in false position control.